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author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Thu, 09 Jul 2015 01:12:16 +0100
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1 <html lang="en">
2 <head>
3 <title>FFTW MPI Wisdom - FFTW 3.2alpha3</title>
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49 <p>
50 <a name="FFTW-MPI-Wisdom"></a>
51 Next:&nbsp;<a rel="next" accesskey="n" href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks">Avoiding MPI Deadlocks</a>,
52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>,
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54 <hr>
55 </div>
56
57 <h3 class="section">6.8 FFTW MPI Wisdom</h3>
58
59 <p><a name="index-wisdom-380"></a><a name="index-saving-plans-to-disk-381"></a>
60 FFTW's &ldquo;wisdom&rdquo; facility (see <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>) can
61 be used to save MPI plans as well as to save uniprocessor plans.
62 However, for MPI there are several unavoidable complications.
63
64 <p>First, the MPI standard does not guarantee that every process can
65 perform file I/O (at least, not using C stdio routines)&mdash;only the
66 process of rank <code>MPI_IO</code> can in general (unless <code>MPI_IO ==
67 MPI_ANY_SOURCE</code>). (In practice, <code>MPI_IO</code> is commonly assumed to
68 be zero, i.e. at least process 0 can perform I/O, since otherwise a
69 single-process run could not perform I/O.) So, if we want to export
70 the wisdom from a single process to a file, we must first export the
71 wisdom to a string, then send it to process 0, then write it to a
72 file.
73 <a name="index-MPI_005fIO-382"></a><a name="index-MPI_005fANY_005fSOURCE-383"></a>
74 Second, in principle we may want to have separate wisdom for every
75 process, since in general the processes may run on different hardware
76 even for a single MPI program. However, in practice FFTW's MPI code
77 is designed for the case of homogeneous hardware (see <a href="Load-balancing.html#Load-balancing">Load balancing</a>), and in this case it is convenient to use the same wisdom
78 for every process. Thus, we need a mechanism to synchronize the wisdom.
79
80 <p>To address both of these problems, FFTW provides the following two
81 functions:
82
83 <pre class="example"> void fftw_mpi_broadcast_wisdom(MPI_Comm comm);
84 void fftw_mpi_gather_wisdom(MPI_Comm comm);
85 </pre>
86 <p><a name="index-fftw_005fmpi_005fgather_005fwisdom-384"></a><a name="index-fftw_005fmpi_005fbroadcast_005fwisdom-385"></a>
87 Given a communicator <code>comm</code>, <code>fftw_mpi_broadcast_wisdom</code>
88 will broadcast the wisdom from process 0 to all other processes.
89 Conversely, <code>fftw_mpi_gather_wisdom</code> will collect wisdom from all
90 processes onto process 0. (If the plans created for the same problem
91 by different processes are not the same, <code>fftw_mpi_gather_wisdom</code>
92 will arbitrarily choose one of the plans.) Both of these functions
93 may result in suboptimal plans for different processes if the
94 processes are running on non-identical hardware. Both of these
95 functions are <em>collective</em> calls, which means that they must be
96 executed by all processes in the communicator.
97 <a name="index-collective-function-386"></a>
98 So, for example, a typical code snippet to import wisdom from a file
99 and use it on all processes would be:
100
101 <pre class="example"> {
102 int rank;
103 FILE *f;
104
105 fftw_mpi_init();
106 MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
107 if (rank == 0 &amp;&amp; (f = fopen("mywisdom", "r"))) {
108 fftw_import_wisdom_from_file(f);
109 fclose(f);
110 }
111 fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD);
112 }
113 </pre>
114 <p>(Note that we must call <code>fftw_mpi_init</code> before importing any
115 wisdom that might contain MPI plans.) Similarly, a typical code
116 snippet to export wisdom from all processes to a file is:
117 <a name="index-fftw_005fmpi_005finit-387"></a>
118 <pre class="example"> {
119 int rank;
120 FILE *f;
121
122 fftw_mpi_gather_wisdom(MPI_COMM_WORLD);
123 MPI_Comm_rank(MPI_COMM_WORLD, &amp;rank);
124 if (rank == 0 &amp;&amp; (f = fopen("mywisdom", "w"))) {
125 fftw_export_wisdom_to_file(f);
126 fclose(f);
127 }
128 }
129 </pre>
130 <!-- -->
131 </body></html>
132